The present invention generally relates to heating systems. More particularly, the present invention relates to a system and method for elevating the deionized water temperature for on-demand use while maintaining water purity and reducing consumption due to trickle flow.
In the biomedical, medical, pharmaceutical, and microchip process sensitive industries, there is a present need to heat ultra pure fluids, such as deionized water. For example, during the manufacture of computer microchips, acids may be used for etching the microchips, and water may be used for rinsing. Because of the small scale of today's micro-circuits and the high manufacturing tolerances required, virtually any impurity in the etching or rinsing fluid can result in defective parts and wasted resources.
It is well-known that the semi-conductor industry uses large amounts of water. A large portion of that water is used to produce deionized (DI) or ultra pure water. In the manufacturing process, DI water is used to rinse and clean semi-conductor wafers. Approximately 1,500 gallons of city water are required to produce 1,000 gallons of DI water. It is estimated that over 2,000 gallons of DI water is required in the production of one 200-mm wafer. Large manufacturing facilities can use approximately three million gallons of DI water per day.
Aside from the cost of the city water itself, producing DI water involves ultraviolet lamps, filters, pumps and recirculating systems which require energy to operate. It is also costly to remove effluents via a water-treatment plant. It is estimated to cost approximately US$12.00 to generate 1,000 gallons of DI water. Reducing DI water consumption by only 1,000 gallons lowers power use by 46 kWh. By reducing DI water use by 10%, a facility using three million gallons of DI water daily can save 5 million kWh annually, or about US$225,000, at an energy price of US4.5¢ per kWh, as well as the savings costs which would otherwise be required for disposal of the wastewater.
After the DI water is produced, it is distributed to the pertinent equipment requiring it by distribution piping that travels throughout the manufacturing facility. Standard equipment requiring DI water include wet benches and process tools. It is often the case that the DI water used in process tools and wet benches is heated for better cleaning results. This is especially the case in rinsing steps that occur after a process involving heated chemistries such as SC1 (typically between 75°-80° C.) or Nitride Etch (typically between 150°-180° C.). Industry has found that the benefits of using heated DI water can include better cleaning results, less water consumption, and reduced potential of stresses to the wafer caused by thermal shock. Therefore, when heated water is used, the DI water travels through a DI water heater prior to reaching the wet bench or process tool.
A common plumbing practice is to configure the wet bench and process tool to allow a continuous or idle flow of the DI water. This is sometimes referred to as a “trickle” flow. The trickle flow is a small continuous DI water flow allowed through the heater and tool and into the drain. Dump rinsers, which are used to rinse the product after an etch operation, are configured to perform “idle” flow, a continuous DI water overflow of the dump rinser. The “trickle” flow and “idle” flow are used to prevent particle and bacteria growth in the heater, tool and/or distribution line. The idle or trickle flow prevents stagnant water or dead legs. Since the trickle flow is continuously flowing into the drain, a large amount of the DI water consumption in a fabrication facility can be attributed to this practice.
Another practice that results in DI water waste is the wait time for equipment to reach the correct temperature as production yields and results have been found to vary with temperature variations. Since temperature is a critical factor in semi-conductor manufacturing, the user must insure that correct temperatures are obtained prior to using the DI water. Therefore, when the DI water is first demanded from a heater, the heater will need to warm up and get to temperature. The DI water is directed into a drain until the proper temperature is reached.
There do exist water heaters which are capable of heating the DI water to the processing temperature in a single pass. For example, Ethridge (U.S. Pat. No. 4,835,365) describes a typical DI water heater that uses PFA (Perflouroalkoxy) Teflon jacketed resistant heaters. By maintaining all wetted parts to be non-contaminating PFA or PVDF (Polyvinydene Fluoride) and designing the system to operate as a single pass system, it is able to provide high purity water at elevated temperatures.
Yane et al., (U.S. Pat. No. 5,930,458) describes another prior art ultra-pure fluid heater that can be used to heat DI water. The heater system offers the same benefits as Etheridge, except Yane utilizes quartz instead of Teflon to isolate the resistance heater from the DI water.
However, these heaters also require trickle flow for the systems at periods when the user does not require DI water. If a trickle is not present, then the DI water in the heater becomes stagnant and the system becomes a dead leg with potential for bacteria and particle build-up. Accordingly, both systems require wait time for the DI water output to reach the desired temperature.
Accordingly, there is a continuing need for a system that heats DI water to an elevated temperature for processing needs in a single pass. What is further needed is a system that reduces or even completely eliminates trickle flow and the costs associated therewith. The present invention fulfills these needs and provides other related advantages.